Patent GB-762106-A (GB)
Title: Improvements in or relating to travelling wave tubes
Inventor: ROGERS DOUGLAS CECIL
IPC: G05D23/12
Filed: 1953-10-14
Abstract: 762,106. Travelling-wave tubes. STANDARD TELEPHONES & CABLES, Ltd. Oct. 14, 1953, No. 28268/53. Class 39(1). A travelling-wave tube comprises a unitary assembly of gun helix and coupling means inserted within the envelope and sealed off by sealing together two annular metal end cap members at the electron gun end of the tube. The tube which is described in detail with reference to Figs. 2 and 4 (not shown) is generally similar to the construction described in copending Specification 762,107. In the exploded view of Fig. 3, the tube has ferromagnetic end caps 39, 40 carrying the collector and gun respectively. End cap 40 comprises a ring 46 sealed to the glass envelope and a ring 47 carrying a glass insert through which the terminal pins and locating spigot are sealed. The pole piece 49 houses the gun and is secured by three ceramic rods 53 to inner choke and pole piece member 54 of ferromagnetic material. Further choke members 57, 58 co-operating wit h brass bushes 103, Fig. 5, are secured to the latter assembly by two pairs of rods 59, 60 which also function as shorting bars for the waveguides U-shaped probes 61 are connected to the ends of the helve 55 which is carried in accurately dimensioned glass tube 56. Collector 62 has a tubular extension 63 which engages with a mica washer 93, Fig. 5, mounted in choke member 54, and a member 64 carrying copper exhaust tube 65. After processing tube 65 is pinched off and heat radiator 43 is secured over tube 64, as shown in Fig. 5. The pole piece face 51 has seatings for three end caps which are welded to ceramic rods 53 as described in copending Specification 762,105. The cupshaped and ring member of cap portion 47 are hard soldered together before sealing in glass base 48, and end cap members 46, 47 are subsequently sealed together by a rapid brazing or welding operation termed a " flash braze " The complete assembly is shown in Fig. 5, the pole piece 49 and ferromagnetic members 88, 89 being energised by coil 91 and also forming a choke. The inner surface of 88 is copper plated. The choke is longer than <#>4 but is designed to prevent escape of energy. Coil 91 is much smaller than any of coils 17, and the slight non-uniformity of the magnetic field due to the external magnetic members not ending flush with gun surface 51 is stated to be advantageous for proper beam focussing. The pole piece and choke arrangement at the output end of the tube is similar, or could be of simpler construction. Holes 94 assist in evacuating the tube, and the heat radiator 43 carries projection 95 with keyway 99, Figs. 3, 5 engaged by peg 100 of boss 96. By tightening screw 101 projection 95 is drawn up to abut against boss 96 so that the travelling wave tube is firmly secured in correct position. The inner wall of the envelope 38 has a coating 105 of loose material such as colloidal graphite or evaporated metal to preferentially attenuate undesired modes which in general have field strengths which decrease more slowly radially than the desired mode. In assembly, the gun, helix and choke members are assembled with ceramic rods 53 in a jig and the flanges of caps 79 welded to the gun and choke 54. Helix support tube 56 is slightly under length to allow for thermal expansion, it is a tight fit on one of its supports, as by slitting the support and bending it out slightly, and a good sliding fit at the other end on a similar support. In an alternative construction, Fig. 6, the ceramic rods 53 have flanged sleeves 107 welded to plated areas on the rods and to pole piece 50 and choke 57. One or more of rods 53 is plated over the whole distance between 50, 57 so as to replcae shorting bars 59. The same arrangement may be adopted for chokes 54, 58. The glass envelope 38 and end caps 39, 46 are now assembled in a jig and sealed together. Next the gun, helix and choke unitary assembly is slid into the envelope until mica washer 93 seats on collector member 63, and members 46, 47 are sealed together as described above. A forced air cooling duct 111 may be provided. The elasticity of mica washer 93 takes up longitudinal expansion of the electrode assembly. In a modification a permanent magnet focussing arrangement can be used. The tube may be used as an amplifier, oscillator, modulator or mixer. A prior art travelling- wave tube and magnetic focussing system is described in detail with reference to Fig. 1 (not shown). In this tube the polepiece housing the gun is located by mica washers one of which supports a metal tube carrying a cup-shaped choke member. A hollow metal tube of small diameter surrounding the beam projects from the base of the cup and forms, together with a straight end portion of the helix, a coupling antenna. A similar antenna is provided at the output end of the tube.